The present invention is related to a liquid crystal mixture, a liquid crystal cell for a color liquid crystal display LCD and the use of a dye with a dipole for a liquid crystal mixture.
Liquid crystals and liquid crystal mixtures can be used for a wide area of applications. One of the most popular applications for liquid crystal is the liquid crystal display LCD especially for notebook computers but increasingly also for monitors of desktop computers of stationary systems.
One of the most important advantages of the liquid crystal displays is, in comparison to other monitors, the small size of the displays as well as the flicker free picture they produce that is very convenient for the user and is especially protective for the eyes of the users, especially in case of long term working before the display.
A liquid crystal display comprises an array of liquid crystal cells. Different liquid crystal cells have been developed in the recent years, the most important liquid display cells are the TN cell (twisted nematic cell), the STN cell (super twisted nematic cell), PDLC cells (polymer dispersed liquid crystal cells) etc. Liquid crystal cells normally use nematic liquid crystals, however, also smectic liquid crystals or cholesteric liquid crystals may be utilized.
All the above mentioned liquid crystal materials generally have common characteristics. They provide a rod-like molecular structure, a rigidness of the long axis and dipoles and/or easily polarisable substituents, therefore providing permanent or induced dipoles.
The distinguishing characteristic of the liquid crystalline state is the tendency of the molecule to point along in the same direction, called the director. The tendency of the liquid crystal molecules to point along the director leads to a condition known as anisotropy. This term means that the properties of the material, especially the light transmission properties, depend on the direction in which they are measured. This anisotropic nature of liquid crystal is responsible for the unique optical properties.
In liquid crystal cells and liquid crystal displays the orientation of the molecules can be controlled by applying an electric or a magnetic field to the liquid crystal material or mixture or the liquid crystal cell. The liquid crystal molecules, having a permanent dipole or an induced dipole, tend to orient themselves along the direction of the field.
By applying an electric or a magnetic field to the liquid crystal mixture or the liquid crystal cell, the molecules can be switched generally between two stages or orientations, an xe2x80x9con-statexe2x80x9d, where the liquid crystal cell is transparent in a preferred direction, and an xe2x80x9coff-statexe2x80x9d, where the liquid crystal cell is not transparent in this preferred direction.
Depending on the liquid crystal cell, the transparency is influenced by different effects: with a TN cell (twisted nematic cell) the polarization of the light is influenced within the liquid crystal material, whereas on both sides of the liquid crystal materials polarizing filters are provided. Depending on the applied field, the polarization is influenced or not influenced within the liquid crystal material, therefore light can transmit both polarizing filters in the above mentioned xe2x80x9con-statexe2x80x9d, or cannot transmit through the polarizing filters in the xe2x80x9coff-statexe2x80x9d.
In a PDLC cell (polymer dispersed liquid crystal cell), small liquid crystal droplets are dispersed uniformly in a transparent polymer matrix. The diameters of the liquid crystal micro-droplets are comparable to the wavelength of visible light. Since these diameters are small compared with the film thickness, a light ray emitted in a first direction (being the direction the transmitting will be essential in regarding the function of the liquid crystal cell or display, therefore also referred to as the functional or the preferred direction) will be scattered many times before emerging from the film, provided that the mismatch of refractive index between liquid crystal and polymer is sufficiently large in the xe2x80x9coff-statexe2x80x9d. The film will appear milky white in the absence of an applied field because of the above mentioned scattering. On the other hand, in the xe2x80x9con-statexe2x80x9d, when the director of the individual droplets is aligned with a field, the refractive index of the liquid crystal within the droplets becomes sufficiently close to the index of the polymer matrix material and therefore the liquid crystal material becomes transparent in the above mentioned preferred direction. When the field is removed, the liquid crystal domains recover their initial orientations because of surface forces and because the cavities are not ideal spheres. The degree of xe2x80x9coff-statexe2x80x9d scattering will depend on the amount of refractive index mismatch, size and the number density of the liquid crystal droplets.
In order to enable display of color information, dichroic dyes are incorporated in the liquid crystal materials.
In general, liquid crystals are excellent solvents for especially organic molecules. Therefore, when a small amount of a dye with geometric anisotropy is mixed in a liquid crystal, the dye molecules couple with the anisotropic intermolecular interaction field of the liquid crystal; they tend to arrange in such a way that their long molecule axes align along the liquid crystal director. When a field, preferably an electric field is applied, the orientation of the dye molecules can be switched along the liquid crystal orientation. This phenomenon is known as xe2x80x9cguest-host interactionxe2x80x9d, the respective liquid crystal cell is therefore called guest-host effect cell.
The quality of liquid crystal cell or liquid crystal display is provided on the one hand by a good contrast and a good color, therefore by good optical properties, on the other hand by the response time of the liquid crystal cells, when switching the cell from e.g. the off-state to the on-state or vice versa.
A rise time is usually defined as the time needed for the material or the cell to reach 90% of the on-state transmission when a field is applied. Similar, a decay time is defined as the time needed for the material in the on-state to reach 10% of the on-state transmission, when the field is set to zero. Fast response times are very important, especially for moved pictures but also for standard computer applications with changing pictures.
It is therefore an object of the present invention to provide a liquid crystal mixture and a liquid crystal cell for liquid crystal displays capable of providing color information with very high response times, both short rise times and short decay times.
This object is solved by a liquid crystal mixture according to claim 1, a liquid crystal cell for a color liquid crystal display (color LCD) according to claim 25 and the use of a dye with a permanent dipole for a liquid crystal mixture. Claims 2 to 24, 26 to 32 and 33 to 45 are related to preferred embodiments of the liquid crystal material, the respective cell and to the use of a dye with a dipole.
According to the invention, the liquid crystal mixture that is especially used for liquid crystal cells for a color liquid crystal display comprises a liquid crystal material and a dye, wherein said dye has a dipole. The dye preferably has a permanent dipole, however, an induced dipole will be sufficient for some applications. By the addition of the dye having a dipole, the rise time can be remarkably shortened. This is due to the additional dipole of the dye, directly influenced by the applied field so that the overall xe2x80x9corientating forcexe2x80x9d, realized by the interaction of the applied field and the overall dipole moment of the liquid crystal mixture could be increased. On the other hand, the decay time is hardly influenced, especially not increased by the addition of the dye with a permanent dipole.
A decrease of the response time, especially the rise time, is important for a good optical quality, furthermore, the lower limit of the operating temperature of the display can be decreased, because the switching time will in generally increase with decreasing temperature, limiting the applicability of the liquid crystal mixture and liquid cells or liquid crystal displays. A liquid crystal mixture according to the invention and a liquid crystal cell for colored liquid crystal displays comprising a dye with a dipole therefore both broaden the temperature range, in which the material or mixture and the cell and displays can be used, and provide better optical qualities for the user in standard conditions.
A dye in the meaning of this application is a medium or a molecular unit comprising at least 1 Chromophore, wherein Chromophore has to be interpreted in its widest possible understanding, i.e. any molecular unit being the reason for a coloring effect, either alone or in combination with any other molecular or atomic unit. Normally Chromophore groups are xcfx80-electron systems. One or more Chromophores can be combined to a Chromogene, also being to be interpreted in its widest possible interpretation.
Further the dye according to the invention can comprise additional groups, wherein these groups can also at least influence the coloring effect, however, these additional groups can also be groups or units, which in no way influence the coloring effect of the dye. Such groups can be for example auxochrome groups, as e.g. NR2, OR, COOH, SO3H etc. normally leading to a shift of the color or the wavelength of the color respectively.
Each molecular unit or sub-unit can alone or in combination with other molecular units or sub-units provide the dipole of the dye according to the invention. The dipole of the dye might especially be provided by one or more Chromophores and/or within units or sub-units of the Chromogene. The dipole might also be provided by units (or sub-units) being extensional groups not included in a Chromophore or a Chromogene. These extensional groups can influence or not influence the coloring effect of the dye, as mentioned above.
It is especially possible and preferable to amend existing dyes without a dipole moment by adding or deleting units, molecules and/or atoms and/or ions, to and from the molecules in order to create a dipole moment of the dye. As mentioned above, this might lead to a shift in the colors, however, it is also possible to add or delete units without influencing the coloring effect, therefore also adding the desired dipole to the dye.
It should be noticed that the coloring effect of the dye should be preferably an effect visible for a human being, it can, however, also be a coloring effect in a wavelength range not being visible for human beings. A coloring effect has therefore also to be understand in its widest possible extent, i.e. any shift of wavelength of light, when being transmitted and/or absorbed and/or reflected by the liquid crystal mixture with a dye according to the invention.
Of course the expression dye is also not limited to only one kind of dye, also two or more different dyes might be contained in the liquid crystal mixture according to the invention.
A liquid crystal xe2x80x9cmixturexe2x80x9d in the sense of this invention has to be understood in a general meaning comprising essentially all materials or elements that might be used or useful in a functional mixture or layer. The liquid crystal mixture has therefore to be understood in case of an application in a liquid crystal cell for e.g. a display as the complete film or cell itself. It can comprise only one specific kind of a liquid crystal material and can also comprise different liquid crystal materials as a blend. It can also comprise additional materials not being liquid crystals. Further the liquid crystal mixture in the sense of this invention can comprise e.g. a polymer matrix for comprising liquid crystal droplets, as will be explained in connection with a preferred embodiment hereinafter. The liquid crystals can also be embedded in other materials or molecular groups. Also any kind of interaction between one or more liquid crystal materials and/or other materials or groups or also chemical bindings may occur, still being comprised from the expression liquid crystal mixture.
The dye with dipole, preferably with a permanent electric dipole, functions as a bifunctional dye, providing both color for optical representation and dipole moment for fast response times. The dye further has preferably optical anisotropic properties.
The liquid crystal material used in the liquid crystal mixture can possibly provide an induced electric dipole, realized by a slight rearrangement of electrons and protons in the molecules, when a field is applied, however, preferably the liquid crystal molecules have strong permanent electric dipoles, therefore realizing a higher overall dipole and higher xe2x80x9corientation forcesxe2x80x9d, as explained above. Liquid crystals with ferrroelectric properties are especially preferred.
The resistivity of the liquid crystal material is preferable very high, in order to decrease power consumption of a liquid crystal cell or a liquid crystal display utilizing the inventive liquid crystal mixture. Resistivities are preferably greater than 1012 xcexa9cm, more preferably greater than 1013 xcexa9cm.
In a preferred embodiment the liquid crystal mixture is operable and controllable by the above explained twisted nematic effect (TN effect), wherein as a liquid crystal preferably 4xe2x80x2-n-pentyl-4-cyanobiphenyl, in the following named as 5CB, is used.
In another preferred embodiment the liquid crystal mixture comprises liquid crystal droplets that are dispersed uniformly in a transparent polymer matrix forming a polymer dispersed liquid crystal (PDLC), as also explained above.
A preferred material is a liquid crystal blend that is e.g. available from the company Merck Limited, Great Britain, under the name xe2x80x9cBL001xe2x80x9d (formally E7), wherein the molecular structure of BL001 is represented by the following formula: 
As polymer material, it is preferred to use a clear, colorless and liquid photopolymer that will cure when exposed to UV light. Such a photopolymer is e.g. available from the company Norland Product Inc. of the United States of America under the name xe2x80x9cNorland Optical Adhesive 65 (NOA65)xe2x80x9d.
The droplets preferably have a size in the range of 0.01 to 20 xcexcm, more preferably between 0.5 and 5 xcexcm. An especially preferred size for a liquid crystal droplet is about 3 xcexcm. Reason for this size is that very large and very small droplets do not scatter light efficiently. An optimum size has been found to be in the area of about 5 times of the wavelength of the light to be transmitted, therefore close to the above mentioned 3 xcexcm. The shape of the droplets is essentially spherical in a preferred embodiment, wherein also some irregularities might occur. However, also other shapes of the droplets are possible, it has to be understood that there are no limitations whatsover regarding the shape of the droplets.
Preferably the inventive liquid crystal mixture is used in a liquid crystal cell operable and controllable according to the guest-host principle, as explained above.
In order to realize a high decrease of the response time, especially the rise time, the dye of the inventive liquid crystal mixture has a permanent dipole of more than 5 debye, preferably of more than 10 debye and most preferably greater than or equal to 15 debye. Of course dyes with higher dipoles can also be utilized in accordance with the present invention.
Preferably the liquid crystal mixture has an order parameter of above 0.5, more preferably above 0.7. The order parameter is a parameter giving a quantitative measurement for the order of the mixture. The order parameter S is defined as follows:   S  =                    A        ||            -              A        ⊥                            A        ||            +              A        ⊥            
wherein A|| and Axe2x8axa5 are the measured absorbance values for the cell in the switched off and switched on state, respectively.
The dye in the inventive liquid crystal mixture has preferably a high solubility, preferably above 0.5 wt. % and more preferably above 1 wt. %.
The liquid crystal material preferably comprises dye in an amount of the sub-percent range until about 5 wt. %, preferably between 0.5 and 2 wt. %, most preferably about 1 to 1.5 wt. %. The wt. % are measured on the basis of the whole liquid crystal mixture including all possible ingredients, also possibly non liquid crystal groups or units or materials, and also including the dye.
The most preferred dye for the inventive liquid crystal mixture is Morpip with a C22H28N4O composition. The formula structure of Morpip is as follows: 
Morpip has a dipole moment of 15+/xe2x88x921 debye in chloroform (depending on the polarity of the solvent) and has an absorption peak at 454.5 nm and is therefore extremely suitable for the inventive liquid crystal mixture.
A suitable general formula for preferred dyes is represented by the following: 
wherein xe2x80x9cDxe2x80x9d represents an electron donating group, xe2x80x9cxcfx80xe2x80x9d represents a pi conjugated system and xe2x80x9cAxe2x80x9d represents an electron acceptor group. Examples of electron donating groups include dialkyamino, pyridino, pyrimidino etc. Examples of electron acceptor groups include nitro, dicyanomethanide, tricyanomethanide etc. Examples of pi systems include all conjugated systems and may include thiophene ring units and phenyl units as well as ethylenic units. All dyes of the following two general formulae are included as examples of the general type; 
wherein D and Dxe2x80x2 represent suitable electron donating groups and Y can represent either hydrogen or halogen. If halogen then Y is preferably fluorine.
Other examples of molecules with dipole moments higher than 10 D are known. A further example is as follows, the following dye has a dipole moment of 17 D. 
The liquid crystal mixture preferably provides a dichroic ratio of at least 2, more preferably of at least 5, most preferably of at least 7 or even higher. The dichroic ratio is defined as the ratio between the intensity of the transmitted light in an on-state and the transmission of light in an off-state of the liquid crystal mixture or cell, measured in the functional direction. A high dichronic ratio will increase the optical properties of respective devices utilizing the inventive liquid crystal mixture.
The invention also relates to a liquid crystal cell for a color liquid crystal display (color-LCD) comprising substrates, where the liquid crystal mixture can be positioned on or between, electrodes for applying an electric field, a voltage source for applying voltage to the electrodes and a liquid crystal mixture comprising a dye with a permanent dipole as explained above. Of course, the voltage source can be an external source, whereas it is also possible that multiple cells e.g. in a display are connected to a single voltage source.
The advantages of such a liquid crystal cell for LCDs, especially the shorter response times, have been already explained above.
Preferably the substrates are manufactured of glass and/or plastic, whereas the electrodes are realized by a conductive coating on the substrates.
The coating is preferably an ITO coating (indium tin oxide coating), wherein the coating is normally provided at one side of the glass of plastic substrates.
A liquid crystal mixture film between the substrate plates, especially the polymer dispersed liquid crystal mixture film, has preferably a thickness between 5 and 40 xcexcm, especially preferably between 10 and 25 xcexcm, wherein this thickness is measured in the functional direction, as explained above.
It shall be noticed that the term substrate plates does not only comprise planar plates but any kind of substrate structure leading to a cell, whereas the liquid crystal mixture is contained within the cell. The substrate plates or simply substrates therefor comprise all realizations for means containing said liquid crystal mixture.
The invention further relates to the use of a dye with a dipole, preferably a permanent dipole liquid crystal cells and/or LCDs, as mentioned above. As also already explained, the most relevant and preferred liquid crystal cells for LCDs are anti-parallel birefringent cells, TN-effect cells, PDLC cells and guest-host cells (GH cells).
It has to be understood that all features explained above and applicable for the liquid crystal mixture according to the invention and especially the dye with a dipole are also relevant for the inventive use of said dye with a dipole.